I dont know sigma driver very well so ll make some speculations
1. The current driver is based on Sigma SDK. The Sigmas one is based on the same SDK but much more older version. So Im expecting that the current driver has more functionality.
2. The current driver is using hotplug to load microcode into chipset. You can extract microcode from windows driver and use it very well. The Sigmas driver is using old version of microcode, so Im not sure that it can play divx3 or maybe something else.
3. The structure of this driver is modular, so It is possible implement drivers for all PCI-board from Sigma with single interface. I have enough reverse-engeneered material to create driver for any Sigmas board (like NetStream, Ventura2000 etc) but I dont have that hardware and I cant test board-specific code. This is the only reason why the drivers for other cards are not made.